Evaporatively cooled traveling-wave tube



May 13, 1969 J. w. HANSEN ETAL 3,444,419

EVAPORATIVELY COOLED TRAVELING-WAVE TUBE Filed Feb. 21, 1967 Sheet of 2 James W Hansen, Leonard T. King, INVENTORS;

ATTORNEY.

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EVAPORATIVELY COOLED TRAVELING-WAVE TUBE Filed Feb. 21, 1967 Sheet 2 v of 2 United States Patent O 3,444,419 EVAPORATIVELY COOLED TRAVELING- WAVE TUBE James W. Hansen, Los Angeles, and Leonard T. King,

Hermosa Beach, Calif., assignors to Hughes Aircraft Company, Culver City, Calif., a corporation of Delaware Filed Feb. 21, 1967, Ser. No. 617,684 Int. Cl. HOlj 25/34 US. Cl. 315-35 Claims ABSTRACT OF THE DISCLOSURE The disclosed assembly includes a hermetically sealed chamber in which a traveling-wave tube is vertically mounted, with a fluid coolant normally existing in the liquid state disposed in the chamber in contact with the electron gun, collector, focusing arrangement, and extended outer surface of an aligning and heat-conveying encasing assembly for the slow-wave structure of the tube. Boiling of the liquid coolant due to heat generated by the tube, causing coolant movement along tube surfaces, and subsequent condensation of the evaporated coolant in a plurality of air-cooled condenser tubes extending upwardly from the chamber enables heat removal from the tube.

BACKGROUND OF THE INVENTION This invention relates to electron beam tubes, and more particularly relates to a traveling-wave tube assembly including an evaporative cooling arrangement.

In traveling-wave tubes a stream of electrons is caused to interact with a propagating electromagnetic wave in a manner which amplifies the electromagnetic energy. In order to achieve such interaction the electromagnetic wave is propagated along a slow-wave structure which is disposed along and about the electron stream path. The slow-*wa've structure provides a path of propagation for the electromagentic wave which is considerably longer than the axial length of the structure, and hence, the traveling-iwave may be made to effectively propagate at nearly the velocity of the electron stream. Interactions between electrons in the stream and the traveling-wave cause velocity modulation and bunching of the stream electrons, the net result being a transfer of energy from the electron stream to the wave traveling along the slowwave structure.

In the design of traveling-wave tubes numerous considerations are involved such as power level, center frequency, bandwidth, operating efiiciency, and tube life, for example. For a particular application, one or more of these considerations are emphasized, although optimization of certain design requirements is usually incompatible with optimization of certain other considerations. For example, when designing a tube for 'wideband operation, it is diflicult to maintain a substantially constant gain and high efliciency over the entire bandwidth; or, when designing a tube for high power operation, it is difficult to maintain long tube life and high reliability.

SUMMAKRY OF THE INVENTION It is an object of the present invention to provide a traveling-wave tube which is capable of operating with a substantially constant gain throughout a relatively wide bandwidth at frequencies near the upper end of the microwave spectrum, and at the same time is able tooperate at substantially higher power levels than prior art tubes of comparable frequencies and bandwidths.

It is a further object of the invention to provide a traveling-wave tube which, in addition to possessing the foregoing advantages, also has a long life and high reliability.

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A traveling-wave tube assembly in accordance with the invention includes a hermetically sealed housing defining a chamber and a plurality of spaced conduits communicating with and extending upwardly from the chamber, with a plurality of cooling fins extending outwardly from the outer surface of each conduit.

A traveling-wave tube subassembly including an electron gun which launches a stream of electrons along an axial path toward a collector is disposed in the chamber in an orientation such that the electron stream travels vertically upwardly. A slow-wave structure is coaxially disposed about the electron stream path between the electron gun and the collector. An encasing arrangement having an extended outer surface is disposed about the slow-wave structure in order to maintain the slow-wave structure in precise coaxial alignment with the electron stream path and in order to convey heat outwardly from the slow-wave structure. A focusing arrangement substantially coextensive with the slow-wave structure is disposed about the slow-wave structure in order to focus the electron stream traveling axially within the slow-wave srtucture, the inner surface of the focusing arrangement being spaced from the extended outer surface of the slow-wave structure encasing arrangement.

A cooling fluid is disposed in at least the lower portion of the chamber in contact with the electron gun, the collector, the slow-wave structure encasing arrangement and the focusing arrangement. The cooling fluid and the pressure within the hermetically sealed housing are selected such that the fluid exists in the liquid state under quiescent conditions but is capable of being vaporized by heat generated by the traveling-wave tube subassembly during operation thereof.

BRIEF DESCRIPTION OF THE DRAWING In the accompanying drawing:

FIG. 1 is a longitudinal sectional view of a travelingwave tube assembly in accordance with the present invention;

FIG. 2 is a cross-sectional view taken along line 2--2 of FIG. 1;

FIG. 3 is a cross-sectional view taken along line 33 of FIG. 1; and

FIG. 4 is an enlarged cross-sectional view taken along line 4-4 of FIG. 1 and illustrating a portion of the assembly including the slow-type structure and its encasing arrangement.

DESCRIPTION OF A PREFERRED EMBODIMENT Referring to FIG. 1 with greater particularity, a traveling-wave tube assembly according to the invention may be seen to include a housing 10, of aluminum for example, which defines a cylindrical chamber 12 in which a traveling-wave tube subassembly 14 is located. The travcling-wave tube subassembly 14 includes an electron gun 16, which may be of a conventional construction wellknown in the art, which functions to launch a stream of electrons along an axial path through the subassembly toward a collector 18. The collector 18 may also be of conventional construction and may include a plurality of outwardly extending flanges 19 to provide an extended coolant contact surface area. The subassembly 14 is oriented such that the electron gun 16 is located in the lower portion of the chamber 12 and the collector 18 in the upper portion. Thus, the electrons travel vertically upwardly from the gun 16 to the collector 18.

As may be seen from FIG. 4, the subassembly 14 also includes a slow-wave structure 20, such as an electrically conductive helix of copper-plated molybdenum for example, which is coaxially disposed about the electron stream between the electron gun 16 and the collector 18.

The slow-wave structure 20 is supported by a plurality of longitudinally extending rods 22, of a dielectric material such as berryllia, equally circumferentially disposed about the slow-wave structure 20. The slow-wave structure 20 and its support rods 22 are mounted within an encasing barrell 24, of copper-plated stainless steel for example. The rods 22 and the slow-wave structure 20 may be held in a rigid clamped relationship within the barrel 24 by employing a barrel triangulating and slow-wave structure assembling technique such as that disclosed in US. Patent No. 2,943,228 to Bernard Kleinman.

In order to ensure precise coaxial alignment of the slow-wave structure 20 with the electron stream path and also to be better facilitate heat removal from the slow- -wave structure, additional encasing means are P ovided externally of the barrel 24. This additional encasing means includes a pair of like elongated blocks 26a and 26b of a material having both high mechanical strength and a high thermal conductivity. Examples of suitable block materials are stainless steel, beryllium-copper, and brass, although brass is preferred on account of its ease of fabrication. Each block 26 defines a longitudinally extending semi-cylindrical groove 28 having a radius slightly greater than the outer radius of the barrel 24. When the blocks 26a and 26b are in the desired assembled relationship as shown, the semi-cylindrical grooves 28 face one another in aligned relationship so as to provide a cylindrical space for receiving the barrel 24. Each block 26 also definies a plurality of longitudinally extending ridges 30 which project outwardly from the block surfaces opposite to the surfaces defining the grooves 28 so as to provide an extended coolant contact surface area.

The blocks 26 are soldered to the barrel 24 by means of a layer 32 of a suitable solder disposed between the outer circumferential surface of the barrel 24 and the grooves 28. Although other solder materials may be used, indium is preferred on account of its relatively low melting point and its desirable thermal expansion properties. In addition to being soldered to the barrel 24, the blocks 26a and 26b are mechanically fastened together in a rigid arrangement, for example by means of screws 34.

Mounted adjacent the outer surface of block 261; is a waveguide 36 which extends parallel to the block 26b throughout substantially its entire length. Appropriate apertures and microwave couplings well-known in the art (but not shown) are provided through the block 26b and the barrel 24 at the end of the waveguide 36 adjacent the electron gun 16 to apply input microwave energy from the waveguide 36 to the electron gun end of the slow-wave structure 20. Similar apertures and momwave couplings are provided through barrel 24 and block 26a adjacent the collector end of the slow-wave structure 20 in order to couple microwave energy from the collector end of the slow-wave structure 20 to a waveguide 38 mounted adjacent the outer surface of block 26a and extending parallel to the block 26a for only a very small portion of its length. The waveguides 36 and 38 both extend upwardly and outwardly from the collector end of the blocks 26 and through the housing for connection to external circuitry (not shown).

In order to focus the electrons traveling from the gun 16 to the collector 18 into a well-collimated beam, an elongated annular focusing arrangement 40 is coaxially disposed about the slow-wave structure 20 for generating a magnetic field along the slow-wave structure axis. The focusing arrangement 40 may include a solenoid 42 in which an electrically conductive foil or tape is wound about a tubular bobbin 44 which is coaxially disposed about the slow-wave structure encasing arrangement so as to leave substantial space between the outer surfaces of blocks 26 and the inner surface of the bobbin 44. The wound tape 42 extends throughout most of the length of the bobbin 44, although short circumferential portions of the bobbin 44 remain exposed at each end. Successive turns of the electrically conductive tape are insulated from one another by an electrically insulating tape which is also wound about the bobbin 44 between successive windings of the conductive tape.

A pair of disk-like annular pole pieces 46 and 48 of ferromagnetic material are disposed at the respective ends of the bobbin 44 and are secured to the bobbin 44, by brazing for example. The ferromagnetic disks 46 and 48 extend radially outwardly from the bobbin 44 beyond the foil solenoid 42. A ferromagnetic tubular member 50 is mounted on and is secured to the outer circumferential surfaces of the pole pieces 46 and 48 to provide an outer return path for the magnetic field generated by the solenoid 42.

In order to bring the magnetic field inwardly to the vicinity of the electron beam, a second pair of disk-like annular ferromagnetic pole pieces 47 and 49 are disposed radially inwardly of the respective pole pieces 46 and 48. The inner pole pieces 47 and 49 are secured, for example by brazing, to opposite ends of the barrel 24 which extend slightly beyond the ends of the blocks 26 The pole pieces 47 and 39 define cylindrical apertures 51 and 53, respectively, in their central regions for passage of the electron beam, the pole piece 49 also being provided with rectangular apertures to accommodate the waveguides 36 and 38.

As may be seen from FIGS. 1 and 3, each outer pole piece 46 and 48 defines a plurality of radially extending slots 52 which register with a like plurality of notches 54 provided in the ends of the bobbin 44. The slots 52 permit cooling fluid to enter and leave the space between the solenoid 42 and the surrounding ferromagnetic members 46, 48 and 50; while the slots 52 in conjunction with the notches 54 permit cooling fluid to enter and leave the space between the blocks 26 and the bobbin 44.

The traveling-wave tube subassemhly 14 is supported within the chamber 12 by means of a plurality of bars 56 which are attached to and which extend radially inwardly from the housing 10 in locations not overlapping the slots 52. The inner ends of the bars 56 are secured to the lower portion of the collector 18 which in turn supports the slow-wave structure 20 and its encasing arrangement. The focusing arrangement 40 (except for the inner pole pieces 47 and 49 which are supported by the slow-wave structure assembly) is supported by means of bolts 57 extending downwardly from the bars 56, the electron gun 16 being supported by the lower inner pole piece 47.

The upper wall for the chamber 12 is defined by a relatively thick plate 60, of aluminum for example, which is secured to an outwardly extending flange 62 of the housing 10 by means of bolts 64 or the like. A sealing ring 66, of rubber for example, is disposed between the flange 62 and the plate 60 in order to provide a hermetic seal for the chamber 12. A plurality of condenser tubes 68 having their longitudinal axes disposed parallel to the longitudinal axis of the cylinder housing 10 and having their lower ends communicating with the chamber 12 extend upwardly from the chamber 12 through the plate 60. A plurality of cooling fins, which may take the form of bristle-like elements 70, extend radially outwardly from the outer circumferential surface of each of the tubes 68 in order to provide an extended surface for the transfer of heat away from the tubes 68. Both the condenser tubes 68 and the cooling fins 70 may be of aluminum, for example.

A tubular housing 72, of aluminum for example, extends upwardly from the plate 60 and surrounds the condenser tubes 68. Enlarged diametrically opposite apertures 74 are provided in the housing 72 so that a coolant, such as air, may be directed through'the space between the outer surfaces of the tubes 68, as shown by the arrows in FIG. 1, in order to facilitate heat removal from the assembly. The coolant may be furnished from a fan or pump (not shown) located externally of the assembly adjacent one of the apertures 74, for example.

A plate 76, of aluminum for example, is secured to the upper end of the housing 72 in order to provide a lower wall for an upper chamber 78. The upper ends of the tubes 68 extend through the plate 76 and communicate with the chamber 78 so as to interconnect the chambers 12 and 78. An annular flange 80 extending upwardly from the outer edge of the plate 76 provides a lateral wall for the cham- 78, while a relatively thick plate 82, also of aluminum for example, is secured to the upper edge of the flange 80 in order to provide an upper wall for the chamber 12. A diaphram safety valve 84 is disposed in an aperture in the wall 82 in order to provide an automatic gas escape path from the chamber 78 in the event the gas pressure within the chambers 78 and 12 exceeds a predetermined level. Also, a manually operable pressure valve 86 is provided through another aperture in the wall 82 to control the gas pressure within the chambers 78 and 12.

Disposed in the lower chamber 12 is a cooling fluid 90, such as a fluorinated hydrocarbon, which at room temperatures fills the chamber 12 to a level 92 slightly below the top of the collector 18. The particular cooling fluid to be used is selected in conjunction with the pressure within the hermetically sealed chambers 12 and 78 such that the fluid exists in the liquid state under quiescent conditions, but is capable of being vaporized by heat generated by the traveling-wave tube subassembly 14 during operation thereof. An example of a particular cooling fluid which appears desirable on account of its electrically insulating and chemically inert properties is an FC-75 fluorinated hydrocarbon sold by Minnesota Mining and Manufacturing Co., St. Paul, Minn.

In preparing the assembly of the present invention for operation, after the traveling-wave tube subassembly 14 has been mounted within the chamber 12, cooling liquid 90 is poured into the chamber 12 up to the level 92. The plate 60 is then placed on and secured to the flange 62 in sealing relationship. The manually operable pressure valve 86 is opened, and the traveling-wave subassembly 14 is energized by applying operating potentials to the electron gun 16, the slow-wave structure 20, the collector 18, and the solenoid 42. After a few minutes the valve 86 is closed, thereby rendering the assembly of the invention in the desired operating condition.

Heat generated by the electron gun 16, the collector l8, and the solenoid 42 is transferred directly to the liquid coolant 90 which is in contact with the outer surface of the electron gun 16 and the collector 18, as well as with both the outer and inner circumferential surfaces and the upper and lower end surfaces of the solenoid 42. Heat generated by the slow-wave structure 20 is conveyed via rods 22, barrel 24, indium layer 28 and blocks 26 to portions of the liquid coolant 90 which contact the extended outer surface of the blocks 26.

As heat is transferred from the traveling-wave tube subassembly 14 to the liquid 90, boiling of the liquid occurs adjacent the heated surfaces of the traveling-wave tube subassembly 14. The resultant vapor bubbles travel to the surface 92 of the liquid enabling fresh liquid to come into contact with the surfaces of the traveling-wave tube subassembly 14 which are exposed to the coolant 90, thereby continually conveying heat away from these surfaces. The vapor escaping from the liquid surface 92 travels upwardly to the condenser tubes 68 where it is cooled by the air passing over the outer surfaces of the tubes 68 and the cooling fins 70. The vapor then condenses on the inner surfaces of the tubes 68, as shown, and returns by gravity to the lower chamber 12.

The assembly of the present invention enables traveling-wave tube operation with a substantially constant gain throughout a relatively wide bandwidth at frequencies near the upper end of the microwave spectrum, and at substantially higher power levels than prior art tubes of comparable frequencies and bandwidths. For

example, a traveling-wave tube assembly according to the invention has been operated over an 8.7% bandwidth at K-band frequencies at a power level as high as 250 watts. On the other hand, the highest power level generally achieved with prior art traveling-wave tubes operating at comparable frequencies and bandwidths is around 25 watts.

Although the present invention has been shown and described with reference to a particular embodiment, nevertheless, various changes and modificatons obvious to a person skilled in the art to which the invention pertains are deemed to lie within the spirit, scope and contemplations of the invention.

What is claimed it:

1. An evaporatively cooled traveling-wave tube assembly comprising:

a hermetically sealed housing defining a chamber and a plurality of spaced conduits extending upwardly from said chamber and each having one end communicating with said chamber, a plurality of cooling fins extending outwardly from the outer surface of each of said conduits;

a traveling-wave tube subassembly disposed in said chamber and comprising: electron gun means disposed at one end of said subassembly for launching an electron stream along an axial path, collector means disposed at the other end of said subassembly for intercepting electrons of said stream, a slowwave structure coaxially disposed about said axial path between said electron gun means and said collector means, encasing means having an extended outer surface disposed about said slow-wave structure for maintaining said slow-wave structure in precise coaxial alignment with said electron stream path and for conveying heat outwardly from said slow-wave structure, focusing means disposed about and substantially coextensive with said slow-wave structure for focusing said electron stream, the inner surface of said focusing means being spaced from said extended outer surface of said encasing means;

means for supporting said subassembly in said chamber in an orientation such that said electron stream travels vertically upwardly;

and a cooling fluid disposed in at least the lower portion of said chamber in contact with said electron gun means, said collector means, said encasing means and said focusing means, said cooling fluid and the pressure within said hermetically sealed housing being selected such that said fluid exists in the liquid state under quiescent conditions but is capable of being vaporized by heat generated by said traveling-wave tube subassembly during operation thereof.

2. An assembly according to claim 1 wherein said chamber is of a cylindrical shape, said conduits are tubes having their respective longitudinal axes disposed parallel to the longitudinal axis of said cylindrical chamber, and said cooling fins are bristle-like elements extending radially outwardly from the outer circumferential surfaces of said tubes.

3. An assembly according to claim 1 wherein said hermetically sealed housing further defines an upper chamber disposed above and communicating with the respective other ends of each of said conduits, and wherein an automatic pressure safety valve and a controllable pressure valve are provided in a Wall of said upper chamber.

4. An assembly according to claim 1 wherein means are provided for directing a coolant through the space between the outer surfaces of said conduits.

5. An assembly according to claim 1 wherein said encasing means includes: a plurality of dielectric support rods equally circumferentially disposed about said slow-wave structure, an essentially tubular member clamped about said rods, a pair of like elongated blocks of a material having a high thermal conductivity disposed about said tubular member, each of said blocks defining a longitudinally extending semi-cylindrical groove having a radius slightly greater than the outer radius of said tubular member, said blocks being mechanically fastened together in a rigid arrangement such that the semi-cylindrical grooves face one another in an aligned relationship to provide a cylindrical space for receiving said tubular member, and a layer of solder material disposed between and soldered to the outer circumferential surface of said tubular member and the surface of said blocks defining said cylindrical space.

6. An assembly according to claim wherein said solder material is indium.

7. An assembly according to claim 5 wherein each of said blocks further defines a plurality of longitudinally extending ridges on a surface opposite to the surface defining said groove.

8. An assembly according to claim 1 wherein said focusing means includes: elongated annular means coaxially disposed about said slow-wave structure for generating a magnetic field along said axial path, a pair of ferromagnetic annular disks coaxially disposed about said axial path and spaced from the respective ends of said elongated annular means, each said ferromagnetic annular disk extending radially outwardly beyond said elongated annular means and defining a plurality of radially extending slots therethrough, and a ferromagnetic tubular member mounted on said ferromagnetic annular disks and spaced from and encircling said elongated annular means.

9. An assembly according to claim 1 wherein said focusing means includes: a non-magnetic electrically conductive tubular member coaxially disposed about said slow-wave structure, a foil solenoid having a length slightly less than the length of said non-magnetic tubular member wound about said non-magnetic tubular member, a pair of ferromagnetic annular disks mounted on respective ends of said tubular member and spaced from the respective ends of said foil solenoid, each said ferromagnetic annular disk extending radially outwardly beyond said foil solenoid and defining a plurality of radially extending slots therethrough, each end of said non-magnetic tubular member defining a plurality of notches registering with respective ones of said slots, and a ferromagnetic tubular member mounted on said ferromagnetic annular disks and spaced from and encircling said foil solenoid. I

10. An assembly according to claim 1 wherein said cooling fluid is a fluorinated hydrocarbon.

References Cited UNITED STATES PATENTS 3,246,190 4/1966 Boyd et a1. 3l53.5 X 3,306,350 2/1967 Beurtheret 165-105 3,388,740 6/1968 OLoughlin 165105 HERMAN KARL SAALBACH, Primary Examiner.

SAXFIELD CHATMON, JR., Assistant Examiner.

US. Cl. X.R.

mg UNITED STATES PATENT OFFICE" i CERTIFICATE OF CORRECTION Patent No. 3 I 444 r 419 Dated ay 13 I 1969 Inventor(s) J. W. Hansen et al It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

601. 2, line 47, "slow-type" should be slow-wave--. Col. 3, line 3, "berryllia" should be beryllia-;

line 29, "definies" should be -defines. Col. 4, line 23, "39" should be 49. C01. 6, line 14, "it" should be -is-.

SIGNED AND SEALED MAR 3 1970 (SEAL) Am Mil-WI:-

lu 0mm I. scam. J Oomissiom of Patents 

